Extreme ultraviolet light, e.g., electromagnetic radiation having wavelengths of around 50 nm or less (also sometimes referred to as soft x-rays), and including light at a wavelength of about 13.5 nm, can be used in photolithography processes to produce extremely small features in substrates, e.g., silicon wafers.
For these processes, it is typically convenient to irradiate the flat workpiece, e.g., wafer, while the workpiece is oriented horizontally. Indeed, orienting the workpiece horizontally may facilitate handling and clamping of the workpiece. This workpiece orientation may then drive the orientations and positions of the scanner optics, e.g., projection optics, masks, conditioning optics, etc., and in some cases may establish a preferential orientation of the initial light beam generated by lithography tool's light source. It is, of course, also generally preferable to minimize the number of optics along the path between the light source and wafer, as each optic reduces light intensity and has the potential to introduce aberrations into the beam. With this in mind, it may happen that a light source which generates a beam of light at a substantial incline to the horizontal direction, is preferable in some instances.
Methods to produce a directed EUV light beam include, but are not necessarily limited to, converting a material into a plasma state that has at least one element, e.g., xenon, lithium or tin, with one or more emission lines in the EUV range. In one such method, often termed laser-produced-plasma (“LPP”), the required plasma can be produced by irradiating a target material having the required line-emitting element, with a laser beam.
One particular LPP technique involves generating a stream of target material droplets and irradiating some or all of the droplets with laser light pulses, e.g. zero, one or more pre-pulse(s) followed by a main pulse. In more theoretical terms, LPP light sources generate EUV radiation by depositing laser energy into a target material having at least one EUV emitting element, such as xenon (Xe), tin (Sn) or lithium (Li), creating a highly ionized plasma with electron temperatures of several 10's of eV. The energetic radiation generated during de-excitation and recombination of these ions is emitted from the plasma in all directions. In one common arrangement, a near-normal-incidence mirror (often termed a “collector mirror”) is positioned at a relatively short distance, e.g., 10-50 cm, from the plasma to collect, direct (and in some arrangements, focus) the light to an intermediate location, e.g., a focal point. The collected light may then be relayed from the intermediate location to a set of scanner optics and ultimately to a wafer. To efficiently reflect EUV light at near normal incidence, a mirror having a delicate and relatively expensive multi-layer coating is typically employed. Keeping the surface of the collector mirror clean and protecting the surface from plasma-generated debris has been one of the major challenges facing the EUV light source developers.
In quantitative terms, one arrangement that is currently being developed with the goal of producing about 100 W at the intermediate location contemplates the use of a pulsed, focused 10-12 kW CO2 drive laser which is synchronized with a droplet generator to sequentially irradiate about 10,000-200,000 tin droplets per second. For this purpose, there is a need to produce a stable stream of droplets at a relatively high repetition rate (e.g., 10-200 kHz or more) and deliver the droplets to an irradiation site with high accuracy and good repeatability in terms of timing and position over relatively long periods of time.
In one previously disclosed arrangement, a substantially vertical stream of droplets is generated and directed to pass through one of the two foci of a collector mirror shaped as a prolate spheroid (i.e., a portion of an ellipse rotated about its major axis). With the vertical stream, the mirror may be positioned out of the path of the droplets. However, with this positioning, a cone-shaped EUV output beam is generated that is aligned along or near the horizontal direction. As indicated above, it may be desirable in some circumstances to produce an EUV source output beam that is substantially inclined relative to the horizontal direction.
Additionally, vertically-oriented droplet streams and the supporting devices may result in vertically-oriented obscurations of the beam path between the collector mirror and the workpiece, e.g. wafer. For some scanner designs non-vertical obscurations may be favored over vertically-oriented obscurations for one or more reasons such as to align the droplet related obscuration with a pre-existing scanner obscuration and/or to produce an obscuration aligned relative to the scan direction which will create an intensity variation at the wafer which ‘averages out’ over a scan and can be compensated by dose adjustment.
With the above in mind, applicants disclose systems and methods for target material delivery in a laser produced plasma EUV light source, and corresponding methods of use.